1. Field of the Invention
The present invention relates to a current reference circuit for producing a reference current and more particularly, to a current reference circuit using a proportional to absolute temperature (PTAT) subcircuit and an inverse PTAT subcircuit to produce a reference current with no temperature dependence.
2. Description of the Prior Art
A current reference circuit that enables to provide a reference current with no temperature dependence is typically composed of a current source producing a constant current whose current value increases in proportion to the ambient absolute temperature, which is termed a "PTAT" subcircuit, and another current source producing another constant current whose current value decreases in inverse proportion to the ambient absolute temperature, which is termed an "inverse PTAT" subcircuit. The two constant currents thus produced are added to cancel their temperature coefficients, resulting in the reference current with no temperature dependence.
FIG. 1 shows a conventional current reference circuit of this sort, which is disclosed in IEEE Electronics, 8th July, 1993, Vol. 29, No. 14.
In FIG. 1, two bipolar transistors Q51 and Q52 and a resistor R52 (1 k.OMEGA.) connected to an emitter of the transistor Q52 constitute a Widlar current source. A base and a collector of the transistor Q51 are coupled together. A base of the transistor Q52 is connected to the base of the transistor Q51. An emitter of the transistor Q52 is connected to the emitter of the transistor Q51 through the resistor R52. It is known well that the Widlar current source is a PTAT circuit that produces a constant current having a positive temperature coefficient.
Assuming that a resistor R53 (1 k.OMEGA.) connected to an emitter of a bipolar transistor Q53 has a resistance with no temperature dependency, a current flowing through the transistor Q53 has a positive temperature coefficient equal to that of the thermal voltage V.sub.T, i.e., +3.333 pm/deg. Since the base of the transistor Q53 is connected to the coupled base and collector of the transistor Q51, and the base of the transistor Q52 is connected to the coupled base and collector of the transistor Q51, a current flowing through the transistor Q52 has a positive temperature coefficient of +3.333 pm/deg.
On the other hand, it is known that the base-emitter voltage of a transistor Q54 whose emitter has a resistor R54 (0.2 k.OMEGA.) has a negative temperature coefficient of about -2 mV/deg under the assumption that a current flowing through the transistor Q54 is a constant current with no temperature dependency. Therefore, the base potential of the transistor Q54 increases as the ambient temperature falls, which causes to increase the voltage drop generated by a resistor R55 (0.2 k.OMEGA.) connected to an emitter of a bipolar transistor Q55. Due to this increase of the voltage drop, a current flowing through the resistor R55 rises. Accordingly, the base potential of a transistor Q55 has a negative temperature coefficient whose absolute value is greater than about 2 mV/deg, which is due to the negative temperature coefficient of the transistor Q55 and the increase of the current flowing through the resistor R55, i.e., an emitter current of the transistor Q55. Thus, the base potential of the transistor Q55 has a temperature coefficient greater than about -2 mv/deg and less than about -4 mV/deg.
When the base potential of the transistor Q55 increases in inverse proportion to the ambient temperature, the base potential of a transistor Q56 whose base is connected to the base of the transistor Q55 increases at a temperature coefficient greater than about -2 mv/deg. On the other hand, since a constant current, which is produced by a current mirror circuit composed of bipolar transistors Q58 and Q59, flows through the transistor Q56, the base-emitter voltage of the transistor Q56 has a temperature coefficient of about -2 mV/deg less than the base potential of the transistor Q55. Therefore, the base-emitter voltage increase of the transistor Q56 is less than the base potential increase of the transistor Q55 and as a result, the emitter potential of the transistor Q56 becomes higher than the base potential of the transistor Q55 by the potential increase difference therebetween.
The constant current supplied by the current mirror circuit composed of the transistors Q58 and Q59 flows not only through the transistor Q56 but also through a resistor R56 connected to the emitter of the transistor Q56 and consequently, a voltage drop generated by the resistor R56 is kept constant. In consideration with this, as the emitter potential of the transistor Q56 increases due to the temperature fall, the base potential of a transistor Q57 whose base is connected to the end of the resistor R56 increases, in other words, a collector current of the transistor Q57 increases. This means that the collector current of the transistor Q57 has a negative temperature coefficient.
Assuming that the resistors R54, R55 and R56 have temperature-independent resistances, respectively, the collector current value of the transistor Q57 can be selected to have a temperature coefficient of -3.333 pm/deg by suitably selecting the resistance values of the respective resistors R54, R55 and R56. In this case, since the collectors of the transistors Q52 and Q57 are coupled together, the two collector currents of the transistors Q52 and Q57 are added to each other so that their temperature coefficients are cancelled. This enables to produce a constant current with no temperature dependency through the coupled collectors of the transistors Q52 and Q57.
Even if the resistors R54, R55 and R56 have respective resistance values with some temperature dependency, the sum of the collector currents of the transistors Q52 and Q57 can be temperature-independent by suitably selecting the resistance values of the respective resistors R54, R55 and R56.
The temperature-independent sum current thus generated at the coupled collectors of the transistors Q52 and Q57 is supplied to the transistor Q54 and the resistor R54 through the current mirror circuit composed of the transistors Q58 and Q59As a result, it is confirmed that the above assumption of the current flowing through the transistor Q54 having no temperature coefficient is correct.
The temperature-independent constant current, which is the sum of the collector currents of the transistors Q52 and Q57, is taken out from a bipolar transistor Q60 whose base is connected to the coupled bases of the transistors Q58 and 59 as an output current I.sub.out of the conventional current reference circuit.
As described above, the collector current of the transistor Q52 as the output current of the PTAT subcircuit, which contains the Widlar current source, and the collector current of the transistors Q57 as the output current of the inverse PTAT subcircuit, which contains the transistors Q54, Q55 and Q56 and the emitter resistors R54, R55 and R56, are added to each other, producing the temperature-independent reference current.
With the conventional current reference circuit shown in FIG. 1, the transistors Q55 and Q56 are essential to constitute the inverse PTAT subcircuit and therefore, the number of the vertically stacked transistors becomes large. This means that this circuit cannot be operated at a satisfactorily low supply voltage such as 3 V.